GPS (Global Positioning System) has been known as a conventional positioning system. A satellite (hereinafter, referred to as “GPS satellite”) for emitting a signal used for the GPS (hereinafter, referred to as a “GPS signal”) flies at an altitude of about 20,000 km from the ground. By receiving and demodulating a signal emitted from the GPS satellite, a user can measure a distance between the GPS satellite and the user. Therefore, if there is no obstacle between the ground and the GPS satellite, positioning is possible using the signal emitted from the GPS satellite. Assume, however, use of the GPS in an urban area. It may often the case that the signal emitted from the GPS satellite cannot be received by a location information providing apparatus of the user, hindered by high buildings standing in large numbers. Further, diffraction or reflection of the signal by a building may cause an error in the distance measurement using the signal and, as a result, positioning accuracy is often degraded.
Though a technique has been known to receive in a room a weak GPS signal that has passed through a wall or roof, state of reception is unstable and positioning accuracy is low.
In the foregoing, the positioning using the GPS has been described by way of example. The phenomenon described above, however, is common to positioning systems using satellites in general. The satellite positioning system is not limited to the GPS, and it may include systems such as GLONASS (GLObal NAvigation Satellite System) of Russian Federation and Galileo of Europe, for example.
Technique related to provision of location information is disclosed, for example, in Japanese Patent Laying-Open No. 2006-67086 (Patent Literature 1).
According to the technique disclosed in Japanese Patent Laying-Open No. 2006-67086, however, the reader or writer is unique to the system for providing location information and lacks versatility. In order to prevent interference, it is necessary to restrain transmission output, and therefore, the area in which location information is receivable is limited and it is difficult to obtain location information continuously. Further, a very large number of transmitters are required to cover a wide area.
Further, in connection with obtainment or notification of location information, it is possible to locate a source of signal transmission if a telephone call is made from, for example, a fixed telephone, as the location of the fixed telephone is known in advance. Wide-spread use of mobile phones, however, makes mobile communication more and more common and it becomes increasingly difficult to notify the location information of the caller, unlike in the case of a fixed telephone. On the other hand, concerning emergency call, legislation is prompted toward inclusion of location information in a call from a mobile phone.
A conventional mobile phone having a positioning function obtains location information in a place where a signal from a satellite is receivable, and therefore, it is possible to notify the location of the mobile phone. In a place where it is impossible to receive radio wave, such as in an underground mall or indoors, however, location information cannot be obtained by the conventional positioning technique.
In view of the foregoing, a technique may be considered in which a plurality of transmitters capable of emitting signals similar to the GPS signal are arranged indoors to find the location based on the principle of trilateration similar to the GPS (for example, see Japanese Patent Laying-Open No. 2000-180527 (Patent Literature 2)). Such an approach, however, requires that the transmitters are synchronized in time, increasing the cost of transmitters.
The invention disclosed in Patent Literature 2 relates to a technique of reducing influence of multipath and the like, utilizing the fact that objects that block or reflect radio waves are arranged at prescribed positions with respect to a direction of movement of a receiving terminal for positioning.
Further, Japanese Patent Laying-Open No. 2007-278756 (Patent Literature 3) discloses a technique of simplifying a system configuration for indoor positioning and improving accuracy of the positioning, by controlling transmission power indoors and by transmitting location information in a format compatible with GPS signal indoors, rather than the trilateration mentioned above.